This invention relates to the use of borates in the recausticization of alkali-containing liquors such as are produced in alkali pulping and bleaching processes for wood and non-wood cellulose fiber materials, and more particularly to a method for improving causticizing efficiency in a conventional lime recausticization process through the addition of borate.
There exist a variety of processes which utilize alkali-based chemicals such as sodium hydroxide in the pulping, bleaching or oxidation of wood and non-wood cellulose-based materials (See Kirk-Othmer, xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, Fourth Edition, Volume 20, Pages 493-582). In the conventional alkaline pulping process, white liquor, an aqueous solution of sodium hydroxide (NaOH) and in many cases sodium sulfide (Na2S), is used as cooking liquor to separate the fibers from wood chips in a high-temperature pressurized digester. The spent chemicals are recovered by filtering the pulp, evaporating/concentrating the filtrate (black liquor), burning the concentrated black liquor in a recovery furnace, and causticizing the resulting inorganic residue (smelt) in a causticizing plant to recover sodium hydroxide for re-use at the beginning of the process.
In a conventional causticizing plant, lime is reacted with an aqueous sodium carbonate solution (green liquor) to regenerate sodium hydroxide (white liquor). The lime causticizing process is carried out through three major steps: i) dissolving smelt from the recovery boiler with water in a dissolving tank to produce green liquor that consists of mostly sodium carbonate (Na2CO3) and Na2S, ii) converting the green liquor into white liquor by causticizing it with lime (CaO) in a slaker, clarifiers and a series of causticizers, and iii) drying and calcining the precipitated lime mud (CaCO3) in a lime kiln to produce CaO which is then reused. The reactions involved are as follows:
Lime Slaking: CaO(s)+H2O(l)xe2x86x92Ca(OH)2(s)
Causticizing: Na2CO3(aq)+Ca(OH)2(s)2 NaOH(aq)+CaCO3(s)
Lime Burning: CaCO3(s)xe2x86x92CaO(s)+CO2(g)
There is an inherent problem in the causticizing operation. The causticizing reaction can never go to completion (or 100% reaction), but rather, approaches equilibrium which can be described by the following equation:       K    =                                        f                          OH              -                        2                    ·                                    [                              OH                -                            ]                        2                                                f                          CO              3                              2                -                                              ·                      [                          CO              3                              2                -                                      ]                              =                                                  [                              OH                -                            ]                        2                                [                          CO              3                              2                -                                      ]                          =                                            f                              OH                -                            2                        ·                                                            [                                      Ca                                          2                      +                                                        ]                                ⁡                                  [                                      OH                    -                                    ]                                            2                                                          f                              CO                3                                  2                  -                                                      ·                                          [                                  Ca                                      2                    +                                                  ]                            ⁡                              [                                  CO                  3                                      2                    -                                                  ]                                                          or      K    =                            f                      OH            -                    2                          f                      CO            3                          2              -                                          xc3x97                        K                      SP            -                                          Ca                ⁡                                  (                  OH                  )                                            2                                                K                      SP            -                          CaCO              3                                          
where K is the apparent equilibrium constant, fOHxe2x88x92 and fCO32 are respectively the dimensionless activity coefficient of OHxe2x88x92 ions and CO32xe2x88x92 ions, and KSPxe2x88x92Ca(OH)2 and KSPxe2x88x92CaCo3 are respectively the solubility constant of Ca(OH)2 and CaCO3.
In pulp mills, the degree of completion of the causticizing reaction or causticizing efficiency (CE) is customarily defined as   CE  =                    [        NaOH        ]                              [          NaOH          ]                +                  [                                    Na              2                        ⁢                          CO              3                                ]                      xc3x97    100    ⁢          xe2x80x83        ⁢    %  
where [NaOH] and [Na2CO3] are respectively the concentration of NaOH and Na2CO3 in the liquor, in mol/L, or in g/L (or Lb/ft3) of Na2O. The equilibrium causticizing efficiency is strongly affected by the liquor strength (total titratable alkali or TTA), the hydroxide concentration (active alkali or AA), the sulfide concentration (sulfidity), and to a smaller extent, the liquor temperature. As shown in FIG. 1, the equilibrium causticizing efficiency decreases markedly from 99% for a dilute solution with a TTA of 2 Lb/ft3 Na2O and 0% sulfidity, to 82% for a concentrated solution of 9 Lb/ft3 Na2O TTA and 30% sulfidity.
The equilibrium causticizing efficiency curves in FIG. 1 (from xe2x80x9cPulp and Paper Manufacture, 3rd Edition, Volume 5, Alkaline Pulpingxe2x80x9d, page 568, TAPPI/CPPA, 1989) serve only as a limit for the maximum possible conversion efficiency. In practice, however, the conversion from Na2CO3 to NaOH is lower since it is also controlled by the reaction kinetics. The reaction usually proceeds rapidly in the beginning, then becomes very sluggish after 20 to 30 minutes because of two main reasons: i) as the concentration of OHxe2x88x92 ions increases with time, the reaction equilibrium tends to shift to the left, and ii) as the reaction proceeds, the surface of Ca(OH)2 particles becomes covered with precipitated CaCO3, which hinders the diffusion of CO32xe2x88x92ions from the solution to the core of Ca(OH)2 particles. Due to the equilibrium and kinetic effects, causticizing efficiency is greatly also affected by the liquor temperature, retention time, lime-to-green liquor ratio and lime quality.
The causticizing efficiency of white liquor in kraft pulp mills is typically about 81%, varying from 78 to 83%. Thus, for every kg of NaOH required for the cooking process, there will be approximately 0.23 kg of unreacted or xe2x80x9cuselessxe2x80x9d Na2CO3 circulating in the liquor. This large amount of carbonate deadload necessitates the use of a higher volume of white liquor per unit of wood chips, thus decreasing the production capacity of the wood pulping circuit.
An alternative method of recausticization which does not require the use of lime and the associated lime recovery process but does involve the use of borate was developed in the 1970""s by Jan Janson, a researcher in Finland (U.S. Pat. No. 4,116,759). Janson proposed that sodium carbonate in the smelt could be causticized automatically (xe2x80x9cautocausticizedxe2x80x9d) in the recovery boiler by the addition of borate to the wood pulping circuit, thus eliminating the need for subsequent recausticization by calcium hydroxide and the accompanying lime recovery circuit. However, it was suggested that conventional causticizing with lime is counteracted by the presence of borate and that these alternative methods of causticization were therefore not compatible with each other and could not be combined (Jan Janson, xe2x80x9cAutocausticizing alkali and its use in pulping and bleachingxe2x80x9d, Paperi ja Puu, 61, #8, p. 502, 1979).
Applicants"" copending application Ser. No. 09/238,828 describes and claims an improved method for regeneration of sodium hydroxide through which smelt is autocausticized at a high rate of efficiency in the recovery boiler by the addition of a limited amount of borate such that the mole ratio of boron to carbonate is less than 2:1 and the mole ratio of sodium to boron is greater than 3:1. Sodium hydroxide is then regenerated in solution when the borate-autocausticized smelt is dissolved. Application Ser. No. 09/238,828 also discloses a process wherein sodium hydroxide is regenerated by a combination of partial autocausticization with borate followed by conventional causticization of unreacted sodium carbonate with lime.
The present invention provides a method for improving the causticizing efficiency of an alkaline pulping liquor. In particular it relates to a method for improving the efficiency of converting sodium carbonate to sodium hydroxide by reaction with lime in the presence of borate.